1. Field of the Invention
The present invention relates generally to heating systems, and more specifically, to a small-scale cogeneration system for providing both heat and electrical power.
2. Background Information
Many commercial buildings as well as homes are heated by forced hot air furnaces. These furnaces typically include an oil or gas-fired burner, a heat exchanger, and an air blower or fan. The heat exchanger typically comprises a plurality of passageways through which hot combustion gases flow. The fan is mounted next to the heat exchanger such that cool air may be forced past the heat exchanger and heated. The fan, which is typically powered by an electric motor, also moves the heated air through the building or home via an arrangement of ducts leading to the various rooms. An electric thermostat operably connected to the burner and the fan is often used to control the furnace. The thermostat switches the furnace on (e.g., activates the burner and the fan) whenever the temperature at the thermostat falls below a preselected level. Operation of the furnace brings warm air into the home. When the temperature at the thermostat exceeds another preselected level, the thermostat shuts the furnace off, thereby suspending the flow of heated air.
One of the main disadvantages of such furnaces is their dependence on electricity. As described above, such furnaces generally include an electrically powered fan to move air past the heat exchanger and through the building or home being heated. In addition, electric power is often used to operate the thermostat and to control the burner. For a 100,000 British Thermal Units/hour (Btu/hr.) residential forced hot air furnace, for example, the electric power requirement is typically between 0.5 to 1.0 kilowatts. Annual electric power costs for operating such a furnace are in the range of $75 to $150. Furthermore, if the electricity delivered to the furnace is interrupted for whatever reason, the furnace is rendered inoperable. That is, without electric power, the thermostat, the burner and the motor that drives the fan will not work, thereby stopping the flow of warm air to the space(s) being heated.
Electric power, moreover, is often lost in blizzards or other cold weather storms. The concomitant loss of the furnace's heating ability, during such periods when the demands for heat are large, can have serious consequences. For example, if the power is disrupted for any length of time, the building or home can become so cold as to be uninhabitable. In addition, the temperature in the building or home may fall below freezing, causing water pipes to burst. The resulting water damage can be substantial.
In addition, large-scale steam-powered stations for generating both electric power and heat are known. Many centralized power production facilities, for example, burn coal or oil to generate high pressure/high temperature steam which, in turn, is used to run one or more generators for providing several megawatts of electrical power. This power may then be supplied to a public power grid or within a campus of buildings. The high pressure/high temperature steam may also be used for space heating purposes. That is, remaining heat energy from the steam, after powering the electric generator(s), may also be provided to neighboring buildings. The steam may then be used for space heating purposes within the buildings.
These large-scale systems (i.e., on the order of several megawatts) typically operate on the well-known Rankine steam cycle. To achieve acceptable fuel efficiency levels, steam boilers producing steam at high pressures (e.g., over 500 pounds per square inch) are required. These boilers typically include a relatively large free surface area for separating the vapor phase (i.e., steam) from the liquid phase (i.e., water), and generating a large inventory of high pressure, high temperature water within the boiler. In addition, complex control systems and heavy wall construction boilers are needed to safely manage the steam. Accordingly, the resulting systems are typically quite large in size and demand constant supervision to ensure safe operation. Indeed, an explosion at theses pressures and temperatures can be catastrophic.
Although these systems are adequate for large-scale operation, they are not suitable for use in most residential or small commercial buildings where the electric power requirements are on the order of 1 to 20 kilowatts. First, the need for a large vapor/liquid surface area, large water inventories, and boilers capable of withstanding the high steam pressures and temperatures demands a system far too large and expensive for practical small-scale installations. The American Society of Mechanical Engineers (ASME) code, moreover, prohibits the practical installation of steam boilers operating at these high pressures in residential settings. Additionally, owners of such systems would be unwilling to provide the needed supervision to ensure safe operation. Indeed, there is no system presently available for providing safe and economical delivery of electrical power and heat on a small-scale (i.e., on the order of 2 to 20 kilowatts) using a high pressure steam boiler. Furthermore, no other means of routinely generating both heat and electrical power on a small-scale, such as internal combustion engines, has been widely adopted due to cost and operating difficulties.